A conventional tire air pressure monitoring system, which monitors the air pressure of each tire of a vehicle, is disclosed in WO 97/38869, for example. This tire air pressure monitoring system includes a tire air pressure sensor, a transmitter and a receiver. The tire air pressure sensor and the transmitter are mounted near each tire. Each transmitter transmits a radio signal including a detected tire air pressure, and the receiver receives the radio signals of the transmitters. Thus, the tire air pressure of each tire is monitored based on the radio signals received by the receiver.
The radio signal of each transmitter, that is, a transmission signal transmitted from the transmitter, is likely to propagate in other directions different from the intended direction or in reduced signal strength for some reasons. For example, if the receiver is mounted on a ceiling roof part of the vehicle, there exists a vehicle compartment between the transmitter and the receiver. As a result, the direction of propagation of the transmission signal will be changed or the signal strength of the transmission signal will be reduced by passengers or luggage in the vehicle compartment.
In the conventional tire air pressure monitoring system, the directivity of the receiver is so set that the sensitivity of receiving the transmission signal is most suited in accordance with the position of the transmitter or the like. If the change in the propagation direction or the reduction in the signal strength of the transmission signal arises, the sensitivity of receiving the transmission signal will be reduced and the transmission signal will not be received.